The interaction of N-alkyl-N,N,N-trimethylammonium (CnTMA, n = 6-18) salts (iodides and/or bromides) with model membranes prepared by hydration of egg yolk phosphatidylcholine (EYPC) over aqueous salt solutions has been studied by m-doxyl stearic acid (m-DSA, m = 12 and 16) spin probe method. In disoriented EYPC bilayers the CnTMA salts decrease the orientational order parameter S33 of m-DSA evaluated from the powder pattern ESR spectra. This effect is maximal for C6TMA. In oriented EYPC bilayers prepared by the parallel-beam sputtering method and hydrated over saturated NaCl solution the order parameter S33 calculated from the angular dependence of the nitrogen hyperfine splitting is decreased in the presence of C6TMA. The order parameter S11 obtained from the angular dependence of line positions indicates deviation of m-DSA motion from axial symmetry. C6TMA increases the probability of gauche conformations of the lipid chains by about 13-14%, and decreases the effective energy difference between the trans and gauche conformations by about 420-480 J/mol, at molar ratio of EYPC/C6TMA = 2:1. The angular dependence of linewidths is analysed by employing a theory of spin relaxation based on the strong collision model for molecular reorientations. The correlation time tau 0 of the reorientation of an axis orthogonal to the doxyl ring of 16-DSA is decreased in the presence of C6TMA, while that of 12-DSA is not influenced by it. The ratio of tau 2/tau 0 is increased in the presence of C6TMA for the both spin probes. The results are explained using the free-volume model of the CnTMA-EYPC membrane interaction.